1. Field of the Invention
The present invention relates to keyboards to be used for electronic devices, for example, numerical controllers for controlling machine tools.
2. Description of the Related Art
FIG. 6 is a schematic cross-sectional view depicting a conventional keyboard. A keyboard mounted on an electronic device is subjected to airborne dust around the keyboard, a mist of oil such as a cutting lubricant, and dirt on the hands of an operator during the use. Such dust, oil mist, and dirt inevitably enter spaces around opening portions of the housing for mounting keys of the keyboard, and adhere to the opening portions of the housing as foreign matter. Such foreign matter may affect operability of the keys.
FIG. 5 illustrates a vertical keyboard. The keyboard is used as a means of entering data into a numerical controller for controlling a machine tool.
A control panel 1 of the numerical controller includes a display device 2 and a vertical keyboard 3. The vertical keyboard 3 is used in a position along the vertical direction of the keyboard. In this structure, the foreign matter such as dust tends to adhere or accumulate between a lower internal surface of a key top 4 and a lower internal surface of an opening portion in the housing (see FIG. 6). When the control panel 1 is used in a state where it stands erect, a first space 5 exists between an upper surface of an opening portion in a housing 7 and an upper surface of the key top 4, and a second space 6 exists between a lower internal surface of an opening portion in the housing 7 and a lower internal surface of the key top 4. The key top 4 moves in the first space 5, thereby preventing dust and dirt from adhering or accumulating onto the upper surface of the key top 4. In the second space 6, on the other hand, dust and dirt easily adhere or accumulate onto the side surface of the lower side of the opening portion. Such accumulated dust and dirt moves from a keyboard exterior 8 side to a keyboard interior 9 side over time, and eventually enter the keyboard interior 9.
To solve such a problem, Japanese Utility Model Application Laid-Open No. 02-070322 discloses a technique for making dust prevention sheet openings on a keyboard surface and an outer periphery of a key come in contact with each other to seal the openings when no key is pressed. Japanese Utility Model Application Laid-Open No. 05-343859 discloses a technique in which a dust prevention sheet is accommodated in a housing of a keyboard and when the keyboard is not used, the dust prevention sheet is pulled out of the housing to cover keys.
The above-described prior art techniques seal the spaces by making the openings of the housing and the outer peripheries of the keys mounted in the openings come in contact with each other, or covering the keyboard surface with cloth to prevent dust and dirt from entering the interior of the keyboard.
Unfortunately, the method of making the housing openings and the key outer peripheries come in contact with each other may enhance the adhesion of the housing openings and keys due to the foreign matter, which is, for example, sticky dirt. The method of covering the surface of the keyboard with the cloth cannot be applied during the use of the keyboard and requires the additional preparation of the cloth, increasing the cost.